Diode-pumped fibre lasers have become commercially available over the last few years that can emit several hundred watts of laser radiation in a near diffraction limited beam. These lasers are based on rare-earth doped optical fibres which emit mainly in the fundamental mode. The optical fibres are typically based on large mode area fibres such as those described in U.S. Pat. No. 6,614,975. The large mode area enables the power levels to be increased to levels which would cause optical damage, as well as non-linear effects such as stimulated Raman scattering and stimulated Brillouin scattering, if a truly single mode core were used.
Further increases in the output power can be achieved by combining single-mode laser radiation from several rare-earth doped fibres together. The laser radiation can be combined incoherently (by using wavelength division multiplexing) or coherently.
Coherent combining requires the laser radiation emitted from each rare-earth doped fibre to be coherent, single mode, and aligned in its state of polarisation.
Coherent combination schemes include master oscillator power amplifier (MOPA) designs in which laser radiation from a single master oscillator (or seed laser) is divided into parallel channels, and amplified by amplifier chains. These amplifier chains typically contain several preamplifiers and power amplifiers, each amplifier comprising several meters to several tens of meters of fibre. The length of each chain needs to be balanced to within the coherence length of the laser radiation emitted by the seed laser. Preferably, the balancing requirement is to within one third or even one tenth of the coherence length. Thus the simplest schemes use seed lasers that have long coherence lengths in order to make the balancing easier.
The power emitted by a coherently combined MOPA system can be increased by either increasing the power emitted by each MOPA chain, or by increasing the number of MOPA chains that are combined together. The power emitted by each MOPA chain is in practice limited by non-linear effects such as stimulated Brillouin scattering (SBS), which has a threshold in a typical system of around 100 W, which can be increased to several hundred watts or higher by broadening mechanisms, such as line broadening, thermal broadening, material broadening and using a seed with a broader linewidth. Using a seed laser with a broader linewidth shortens the coherence length of the laser radiation. However, if stimulated Brillouin scattering occurs, it can lead to undesired pulsing of the laser, and very fast almost instantaneous catastrophic damage.
Increasing the linewidth of the seed laser is very desirable in order to increase the power level at which SBS becomes noticeable (the so-called SBS threshold). However, as the linewidth increases, the coherence length decreases, and the balancing requirement becomes more difficult. The amplifiers therefore have to be built to exacting tolerances. This is difficult, especially when the need for maintenance and replacement of amplifiers is taken into account. The amplifiers are preferably spliced together, a process that involves cleaving the fibres (often more than once to ensure perfect end faces) and splicing them together in a commercial fusion splicer, often in the field (as opposed to an air-conditioned clean room).
SBS is characterized by a frequency shift between the forward propagating pump wave and the backward travelling wave. The frequency shift at a pump wavelength of around 1 μm is typically around 10 GHz to 17 GHz in silica optical fibres, the variation being dependent upon dopants and whether the fibre has a solid uniform or non-uniform core or is microstructured. The bandwidth of the Brillouin gain is in the range 35 MHz to 100 MHz. This bandwidth corresponds to the lifetime of the acoustic phonon that is generated in the spontaneous Brillouin scattering process. This bandwidth corresponds to an effective Brillouin scattering length LB of optical fibre of around 6 m to 2 m.
A problem that can occur in high power lasers and amplifiers is when the SBS threshold at a particular power is reached in an effective gain length. LG comparable to or less than LB. For example, for a 1 kW signal, the effective gain length may be of the order of 20 mm. For the linewidth broadening approach, this would require a linewidth many times the Brillouin bandwidth. This would further increase the difficulty of balancing the MOPA chains.
There is therefore a need for a MOPA architecture which eases the requirements for maintenance and repair, and which permits seed lasers having shorter coherence lengths to be used.
An aim of the present invention is to provide an apparatus for combining laser radiation which reduces the above aforementioned problem.
The Invention:
According to a non-limiting embodiment of the invention, there is provided apparatus for combining laser radiation, which apparatus comprises a seed laser, a splitter, a plurality of amplifier chains, a reference amplifier chain, detection means, demodulator means, and phase control means, wherein each of the amplifier chains comprises at least one optical amplifier, optical radiation emitted from the seed laser is split into the plurality of amplifier chains by the splitter, amplified by the plurality of amplifier chains, interfered, detected by the detection means, demodulated by the demodulator means, and signals indicative of path length imbalance fed back to the phase control means, and wherein the apparatus is characterised in that the output power emitted by each amplifier chain is at least 50 W, the bandwidth of the seed laser is at least 1 GHz, and the path length difference between each amplifier chain and the reference amplifier chain is less than the coherence length of the seed laser.
The apparatus may include at least one path length balance means for balancing the path length difference between at least one of the amplifier chains and the reference amplifier chain to within the coherence length of the seed laser. The path length balance means may comprise a length of optical fibre.
The phase control means may comprise a length of optical fibre that is heated. The length of optical fibre may be an amplifying fibre within at least one of the amplifier chains.
The phase control means may utilize the quantum defect in an optical fibre amplifier. The phase control means may be based upon varying the relative proportions of pump power emitted by a longer wavelength pump and a shorter wavelength pump. The phase control means may utilize an additional signal wavelength that is different from the signal wavelength emitted by the seed laser.
The path length difference between each of the plurality of amplifier chains and the reference amplifier chain may be measured by physical length measurements. The path length difference may be measured using optical methods. The path length difference may be measured by inserting a scanning interferometer between the seed laser and the splitter, scanning the interferometer, and analysing the response of the detection means. The path length difference may be measured by measuring the phase excursions of each of the plurality of amplifier chains following application of a phase angle stimulus. The phase angle stimulus may be achieved by turning the apparatus on. The phase angle stimulus may be achieved by chirping the seed laser. The optical path length of the reference amplifier chain may be changed prior to the application of the phase angle stimulus.
The phase control means may include a controller that comprises memory for dynamic balancing of the path length imbalance.
The seed laser may comprise a narrow linewidth laser and a modulator comprising at least one phase modulator. The modulator may be driven with a periodic signal. The modulator may be driven with a random or a pseudo-random signal having a bandwidth less than the stimulated Brillouin frequency shift FSBS. The periodic signal may comprise a sinusoidal, a triangular, a saw tooth, or a square waveform. The periodic signal may comprise a parabolic waveform. The frequency of the periodic signal may be greater than the Brillouin bandwidth. The amplitude of the periodic signal may be greater than π radians. The amplitude may be greater than 10π radians. The product of the amplitude and the frequency of the periodic signal may be greater than 1 GHz. The product of the amplitude and the frequency may be less than the stimulated Brillouin frequency shift FSBS.
The modulator may be driven with a second periodic signal having a frequency greater than the stimulated Brillouin frequency shift FSBS. The frequency of the second periodic signal may be more than approximately twice the stimulated Brillouin frequency shift FSBS. The second periodic signal may be a phase shift keying (PSK) signal.
The modulator may be driven with a phase shift keying (PSK) signal.
The modulator may be driven with a continuous phase frequency shift keying (CPFSK) signal.
At least one of the amplifier chains may be characterised by an effective gain length LG and an effective Brillouin scattering length LB, and the optical power emitted by the amplifier chain may be such that the effective Brillouin gain length LG is comparable or less than the effective Brillouin scattering length LB.
The periodic signal may be characterized by a wavelength, and the wavelength of the periodic signal may be less than the effective Brillouin gain length LG. The wavelength may be less than one ball of the effective Brillouin gain length LG.
According to another non-limiting embodiment of the invention, there is provided a method of balancing the apparatus of the invention, the method comprising applying a phase angle stimulus, and measuring the path length, imbalance between at least one of the amplifier chains and the reference amplifier chain from the signal indicative of path length imbalance. The method may comprise the additional step of misbalancing the reference amplifier chain. Alternatively or additionally, the method may comprise the additional step of misbalancing at least one of the amplifier chains.
According to another non-limiting embodiment of the invention, there is provided a method of balancing the apparatus of the invention, which method comprises providing an interferometer, inserting the interferometer between the seed laser and the splitter, scanning the interferometer, and measuring the path length imbalance between at least one of the amplifier chains and the reference amplifier chain from the signal indicative of path length imbalance. The method may comprise the additional step of misbalancing the reference amplifier chain. Alternatively or additionally, the method may comprise the additional step of misbalancing at least one of the amplifier chains.